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<hr>
<h2 id="RDB（Redis-DataBase）"><a href="#RDB（Redis-DataBase）" class="headerlink" title="RDB（Redis DataBase）"></a>RDB（Redis DataBase）</h2><h3 id="什么是RDB？"><a href="#什么是RDB？" class="headerlink" title="什么是RDB？"></a>什么是RDB？</h3><p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503153738.png" alt="image-20200503153737212"></p>
<p>在指定的时间间隔内将内存中的数据集快照写入磁盘，也就是行话讲的Snapshot快照，它恢复时是将快 照文件直接读到内存里。</p>
<p>Redis会单独创建（fork）一个子进程来进行持久化，会先将数据写入到一个临时文件中，待持久化过程 都结束了，再用这个临时文件替换上次持久化好的文件。整个过程中，主进程是不进行任何IO操作的。 这就确保了极高的性能。如果需要进行大规模数据的恢复，且对于数据恢复的完整性不是非常敏感，那 RDB方式要比AOF方式更加的高效。</p>
<p>RDB的缺点是最后一次持久化后的数据可能丢失。我们默认的就是 RDB，一般情况下不需要修改这个配置！ 有时候在生产环境我们会将这个文件进行备份！ <strong>rdb保存的文件是dump.rdb 都是在我们的配置文件中快照中进行配置的！</strong></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503153846.png" alt="image-20200502180324936"></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503154058.png" alt="image-20200502180554835"></p>
<h3 id="触发机制"><a href="#触发机制" class="headerlink" title="触发机制"></a>触发机制</h3><p><strong>save的规则满足的情况下，会自动触发rdb规则</strong> </p>
<p><strong>执行 flushall 命令，也会触发我们的rdb规则！</strong> </p>
<p><strong>退出redis，也会产生 rdb 文件！</strong></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503154032.png" alt="image-20200503154030474"></p>
<h3 id="恢复数据"><a href="#恢复数据" class="headerlink" title="恢复数据"></a>恢复数据</h3><p>只需要将rdb文件放在我们redis启动目录就可以，redis启动的时候会自动检查dump.rdb 恢复其中的数据！</p>
<p><code>config get dir</code>查看需要存在的位置</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503154406.png" alt="image-20200503154406003"></p>
<h3 id="优缺点"><a href="#优缺点" class="headerlink" title="优缺点"></a>优缺点</h3><div class="note success flat"><p>优点</p>
</div>

<p><strong>适合大规模的数据恢复！</strong> </p>
<p><strong>对数据的完整性要不高！</strong> </p>
<div class="note danger flat"><p>缺点 </p>
</div>

<p><strong>需要一定的时间间隔进程操作！如果redis意外宕机了，这个最后一次修改数据就没有的了！</strong> </p>
<p><strong>fork进程的时候，会占用一定的内容空间！！</strong></p>
<h2 id="AOF（Append-Only-File）"><a href="#AOF（Append-Only-File）" class="headerlink" title="AOF（Append Only File）"></a>AOF（Append Only File）</h2><p><strong>将我们的所有命令都记录下来，history，恢复的时候就把这个文件全部在执行一遍！</strong></p>
<h3 id="什么是AOF？"><a href="#什么是AOF？" class="headerlink" title="什么是AOF？"></a>什么是AOF？</h3><p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503155327.png" alt="image-20200503155326082"></p>
<p>以日志的形式来记录每个写操作，将Redis执行过的所有指令记录下来（读操作不记录），只许追加文件 但不可以改写文件，redis启动之初会读取该文件重新构建数据，换言之，redis重启的话就根据日志文件 的内容将写指令从前到后执行一次以完成数据的恢复工作 </p>
<p><strong>Aof保存的是 appendonly.aof 文件</strong></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503155551.png" alt="image-20200503152325827"></p>
<p>默认是不开启的，我们需要手动进行配置！我们只需要将 appendonly 改为yes就开启了 aof！ 重启redis 就可以生效了！</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503160228.png" alt="image-20200503160227800"></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503160320.png" alt="image-20200503160319265"></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503160418.png" alt="image-20200503160417499"></p>
<p>如果这个 aof 文件有错位，这时候 redis 是启动不起来的吗，我们需要修复这个aof文件 redis 给我们提供了一个工具<code>redis-check-aof</code></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503160815.png" alt="image-20200503160813460"></p>
<p>使用<code>redis-check-aof --fix appendonly.aof</code>进行修复</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503161042.png" alt="image-20200503161041225"></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503161123.png" alt="image-20200503161122840"></p>
<p>如果文件正常，重启就可以直接恢复数据了！</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503161306.png" alt="image-20200503161305172"></p>
<h3 id="重写规则说明"><a href="#重写规则说明" class="headerlink" title="重写规则说明"></a>重写规则说明</h3><p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20200503161702.png" alt="image-20200503161701432"></p>
<h3 id="优缺点-1"><a href="#优缺点-1" class="headerlink" title="优缺点"></a>优缺点</h3><div class="note success flat"><p>优点</p>
</div>

<p><strong>每一次修改都同步，文件的完整会更加好！</strong> </p>
<p><strong>每秒同步一次，可能会丢失一秒的数据</strong> </p>
<p><strong>从不同步，效率最高的！</strong></p>
<div class="note danger flat"><p>缺点 </p>
</div>

<p><strong>相对于数据文件来说，aof远远大于 rdb，修复的速度也比 rdb慢！</strong> </p>
<p><strong>Aof 运行效率也要比 rdb 慢，所以我们redis默认的配置就是rdb持久化！</strong></p>
<h2 id="扩展"><a href="#扩展" class="headerlink" title="扩展"></a>扩展</h2><ol>
<li><p>RDB 持久化方式能够在指定的时间间隔内对你的数据进行快照存储 </p>
</li>
<li><p>AOF 持久化方式记录每次对服务器写的操作，当服务器重启的时候会重新执行这些命令来恢复原始 的数据，AOF命令以Redis 协议追加保存每次写的操作到文件末尾，Redis还能对AOF文件进行后台重 写，使得AOF文件的体积不至于过大。 </p>
</li>
<li><p>只做缓存，如果你只希望你的数据在服务器运行的时候存在，你也可以不使用任何持久化 </p>
</li>
<li><p>同时开启两种持久化方式 在这种情况下，当redis重启的时候会优先载入AOF文件来恢复原始的数据，因为在通常情况下AOF 文件保存的数据集要比RDB文件保存的数据集要完整。 RDB 的数据不实时，同时使用两者时服务器重启也只会找AOF文件，那要不要只使用AOF呢？作者 建议不要，因为RDB更适合用于备份数据库（AOF在不断变化不好备份），快速重启，而且不会有 AOF可能潜在的Bug，留着作为一个万一的手段。 </p>
</li>
<li><p>性能建议 </p>
</li>
</ol>
<ul>
<li>因为RDB文件只用作后备用途，建议只在Slave上持久化RDB文件，而且只要15分钟备份一次就够 了，只保留 save 900 1 这条规则。</li>
<li>如果Enable AOF ，好处是在最恶劣情况下也只会丢失不超过两秒数据，启动脚本较简单只load自 己的AOF文件就可以了，代价一是带来了持续的IO，二是AOF rewrite 的最后将 rewrite 过程中产 生的新数据写到新文件造成的阻塞几乎是不可避免的。只要硬盘许可，应该尽量减少AOF rewrite 的频率，AOF重写的基础大小默认值64M太小了，可以设到5G以上，默认超过原大小100%大小重 写可以改到适当的数值。 </li>
<li>如果不Enable AOF ，仅靠 Master-Slave Repllcation 实现高可用性也可以，能省掉一大笔IO，也 减少了rewrite时带来的系统波动。代价是如果Master/Slave 同时倒掉，会丢失十几分钟的数据， 启动脚本也要比较两个 Master/Slave 中的 RDB文件，载入较新的那个，微博就是这种架构。</li>
</ul>
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